Preamplifiers noise

The most important factors in determining the s/n ratio or sensitivity of an NMR experiment in a cryogenic NMR probe, insofar as the hardware itself is concerned, are the temperature of the coil, 7/ the resistance of the coil, Rc the temperature of the sample, Ts and the resistance added to the coil by the presence of the sample or the sample resistance, Rs. The temperature of the rf coil in a cryogenic NMR probe is typically in the range of 15-30K commercial examples of the Varian Cold-probe operate at 25 K. The preamplifier noise temperature is generally in the range of 10-15 K and the coil resistance is small compared to a conventional room temperature NMR probe. The first two terms in the expression below provide the basis for the vast improvement in the performance of a cryogenic relative to a conventional NMR probe. The sample temperature, Ts, and sample resistance, Rs,... 

Low preamplifier noise, and very high gain ( 10 ) to detect... 

An imaging high-pressure detector can be envisioned from an array of vertically cylindrical ionization chambers, with spatial resolution set by each tube diameter. It may further be possible to segment the collection anode, to derive an azimuthal co-ordinate within each detector and to use signal risetime to get a radial co-ordinate. The precision of such techniques, and the low-energy performance of such detectors is critically dependent upon the preamplifier noise. It may be possible to achieve around 50 electrons rms with modern (optical feedback, or no feedback) amplifiers resulting in an energy resolution of a few percent at 100 keV. 

The dynamic charge restoration feedback differs from the resistive feedback in that a more complicated, active feedback network is substituted for the simple resistor feedback. This results in a lower preamplifier noise at long shaping time constants, but maintains an energy rate capability at short shaping time constants similar to the resistive feedback preamplifier. Pulse shapes at the output of the dynamic charge restoration preamplifier are similar to the pulses from the resistive feedback preamplifier. In both types of continuous feedback there is no deadtime or deadtime loss associated with the preamplifier. 

Figure 4.27 shows the pulsed optical feedback preamplifier of the Landis and Goulding design [28]. It is identical to the resistive feedback preamplifier except that the feedback resistor has been removed. This change permits lower preamplifier noise at long shaping time constants. In place of the feedback resistor an optical reset circuit has been incorporated. 

The total FWHM resolution on a peak at energy E is a combination of the preamplifier noise contribution Fnoise the ionization statistics Fjonization. and other line-broadening contributions Foiher such as incomplete charge collection. The total FWHM resolution is given by... 

Figurs 4.33 The preamplifier noise contribution Fnoise as a function of the amplifier shaping time constant for a small detector [23].

The preamplifier noise is then given by the following equation ... 

Figure 4.11 (a) The variation of preamplifier noise with input capacitance, (b) The measured capacitance of a detector at different bias settings... 

---